Semiconductor workpieces are processed within process chambers. One such chamber is known as a plasma deposition chamber, which is part of a PLAD system. In operation, one or more dopant gasses are fed into the process chamber. These gasses are energized into a plasma through the use of radio frequency (RF) or other forms of energy, such as by utilizing one or more RF antennas or coils. A workpiece is disposed on a platen within the process chamber. This platen may be in electrical communication with a power supply, which can apply a bias voltage to the platen. When the platen is negatively biased, the positively charged species, or ions, from within the plasma accelerate toward the workpiece, thereby implanting the dopant species in the workpiece. At times when the plasma is on but the bias voltage to the platen is in the off state, there may be conditions conducive to dopant deposition on the wafer surface, instead of dopant implantation into the wafer.
Implants performed using PLAD systems typically utilize high concentrations of charged species in the plasma and therefore, perform relatively high dose implants. For example, the dopant concentration implanted in the workpiece using a PLAD system may be between 1E16 and 1E17 ions per square centimeter. This implant may be performed in a relatively short amount of time, such as between 30 seconds and a few minutes. This can be achieved because the concentration of ions within the plasma is typically much greater than that found in an ion beam generated in an ion beam line system.
PLAD systems are also effective for conformal doping applications. These include applications where dopant is to be implanted in all exposed surfaces of a three-dimensional structure. Examples of these structures include raised structures, such as fin type structures, and recessed structures, such as trenches. Unlike beam line systems, PLAD systems are effective at implanting ions into both the vertical surfaces and the horizontal surfaces of the workpiece.
Recently, a new set of applications, such as CMOS image sensors (CIS) shallow trench isolation (STI) and channel doping for threshold voltage control, has arisen, which require conformal doping at ion concentration levels much less than those typically associated with PLAD systems, such as 1E13.
A PLAD system, using present operating parameters, would implant this concentration of charged ions in a workpiece in a very short period of time, such as about 0.5 seconds. This period may be too short to allow adequate process control and guarantee wafer-to-wafer repeatability. Additionally, given the short duration of the implant, the species concentration in the workpiece may not be uniform.
Therefore, it would be beneficial if there were a method of achieving low dose doping and particularly, low dose, conformal doping, of a workpiece using a PLAD system.